Method for fabricating liquid crystal display device

ABSTRACT

A method for fabricating a liquid crystal display device includes: forming alignment keys by at least one of ink jet printing, offset printing, screen printing and laser marking on a dummy region of an upper substrate; forming a first alignment film over an active region of the upper substrate; forming sealant along a periphery of the active region of the upper substrate; and bonding the upper substrate to a lower substrate.

[0001] This application claims the benefit of the Korean Application No.P2003-0042965 filed on Jun. 28, 2003, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to liquid crystal display devices,and more particularly, to a method for fabricating a liquid crystaldisplay device using alignment keys.

[0004] 2. Background of the Related Art

[0005] Keeping pace with the development of an information orientedsociety, demand for display devices in various forms have increased. Tomeet these demands, various types of flat display devices, such as anLCD (Liquid Crystal Display), PDP (Plasma Display Panel), ELD (ElectroLuminescent Display) and VFD (Vacuum Fluorescent Display) devices, havebeen used as display devices in various apparatuses. Of the varioustypes of display devices, the LCD is most widely used for mobile displaydevices due do its advantages of good picture quality, light weight,thin profile, and low power consumption. Besides mobile display devices,such as displays for notebook computers, the LCD has been used intelevisions and as monitors for computers. Using the LCD in variousfields as a general display device increase the need of a high qualitypicture, such as high definition, high luminance, and a large sizepicture, while maintaining the features of light weight, thin profile,and low power consumption. However, recent technical developments in LCDdevices to serve as general display devices by enhancing the picturequality are contradictory to above advantages in many aspects.

[0006]FIG. 1 illustrates a disassembled perspective view of a part of arelated art TN liquid crystal display device, including a lowersubstrate 1, and an upper substrate 2 bonded together with a spacetherebetween, a liquid crystal layer 3 between the lower substrate 1 andthe upper substrate 2. The lower substrate 1 has a plurality of gatelines 4 arranged at regular intervals in one direction, and a pluralityof data lines 5 arranged at regular intervals perpendicular to the gatelines 4, to define a plurality of pixel regions “P.” The upper substrate2 has a black matrix layer 7 for shielding light to parts except thepixel regions ‘P’, RGB color filter layers 8 for displaying colors and acommon electrode 9. The upper and lower substrates are bonded by asealant. A space is maintained between the upper and lower substratesthrough the use of spacers.

[0007] A pixel electrode 6 and a thin film transistor “T” are located ineach of the pixel regions. The thin film transistor “T” has a gateelectrode projecting from the gate line 4, a gate insulating film (notshown) formed over an entire surface of the lower substrate 1, an activelayer over the gate insulating film over the gate electrode, and asource electrode projected from the data line 5, and a drain electrodeopposite to the source electrode. The pixel electrode 6 is formed of atransparent conductive metal having relatively high light transmitivity,such as indium-tin-oxide (ITO).

[0008] The twisted nematic (TN) type LCD can display a picture byorienting the liquid crystal layer 3 on the pixel electrode 6 using asignal applied through the thin film transistor “T” to manipulate theorientation of the liquid crystal layer 3 so as to control the quantityof light transmitting through the liquid crystal layer 3. The TN typeLCD, which is driven by an electric field having an up/down direction,has the good characteristics of high light transmitivity and goodaperture ratio. Further, a TN type LCD is resistant to staticelectricity because the common electrode 9 of the upper substrate 2serves as ground.

[0009] The Related Art LCD has alignment problems. More particularly,the color filter layer on the upper layer is liable to misalign with thepixel region on the lower substrate during the bonding of the upper andthe lower substrates. Such a positional deviation between the pixelregions on the lower substrate and the color filter layer on the uppersubstrate becomes an even greater problem when the substrates are largerand the apertures decrease (i.e. resolution increases). To solve theseproblems, a color filter On TFT (COT) array or TFT array On Color filter(TOC) structure has been proposed.

[0010]FIG. 2 illustrates a sectional view of a related art LCD of a COTstructure and FIG. 3 illustrates a plan view of a related art LCD of aCOT structure. Referring to FIG. 2, the related art LCD of a COTstructure is provided with a lower substrate 20 having an active regionthereon containing pixel regions for displaying a picture. RGB colorfilters 21 are positioned in each of the pixel regions within the activeregion on the lower substrate 20. A sealant 23 is positioned on aperiphery of the active region of the lower substrate 20 to preventleakage of the liquid crystals, and bonding the upper substrate 30 andlower substrate 20.

[0011] An alignment film 22 is formed over an entire surface of thelower substrate 20 and the RGB color filter layers 21. The alignmentfilm 22 is formed larger than the active region with a preset margin“a”, and the sealant is spaced from the alignment film 22 with a presetmargin “b” for preventing the alignment film 22 from peeling. Moreover,although not shown, there are alignment keys formed on the lowersubstrate 20 on an outer side of the sealant 23 formed at the time ofpatterning the gate lines and the data lines. There is also an alignmentfilm 32 in an active region 32 of the upper substrate 30 shown in FIGS.2 and 3.

[0012] To prevent back light leakage from outside of the active region,the upper substrate 30 has a light shielding film 31 on a periphery ofthe upper substrate 30, i.e., around the active region. Alignment keys33 are formed on the upper substrate 30 and the lower substrate 20 on anouter side of the light shielding film 31 for an accurate alignmentduring different processes. The regions of the substrates that thealignment keys are formed on will be cut away after the bonding of theupper/lower substrates. Alignment keys 33 are, for an example, alignmentfilm printing keys, bonding keys, and seal printing keys, and the like.

[0013] Since no black matrix layer is formed on the active region of theupper substrate, the COT structure may have a light shielding film ofresin over a channel region of a thin film transistor on the lowersubstrate to prevent reflection of an external light. In the process forapplying the resin over the channel region of the thin film transistoron the lower substrate, a light shielding film of resin is formed on theouter side of the active region of the lower substrate instead of on theouter side of the active region of the upper substrate. Although astructure is suggested, in which a light shielding film of resin isformed on an outer periphery of the active region of the lower substrateto shield a back light leaking from an outer part of the active region,even if the light shielding film is formed on the lower substrate thus,the fabrication process is not simple because it requires the formationof alignment keys on the upper substrate by a photo-etching process.That is, even if the light shielding film is formed on the outer side ofthe active region of the lower substrate instead of about the peripheryof the active region of the upper substrate, there is a problem in thatthe process for forming the alignment keys on the outer side of theactive region of the upper substrate 30 uses a photo-etching process,which is complicated and requires precision.

SUMMARY OF THE INVENTION

[0014] Accordingly, the present invention is directed to a method forfabricating a liquid crystal display device that substantially obviatesone or more of the problems due to limitations and disadvantages of therelated art.

[0015] An object of the present invention is to provide a method forfabricating a liquid crystal display device, which can simplify aprocess for forming alignment keys.

[0016] Additional features and advantages of the invention will be setforth in the description which follows, and in part will be apparent tothose having ordinary skill in the art upon examination of the followingor may be learned from practice of the invention. The objectives andother advantages of the invention will be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

[0017] To achieve these objects and other advantages and in accordancewith the purpose of the present invention, as embodied and broadlydescribed herein, the method for fabricating a liquid crystal displaydevice includes the steps of: forming alignment keys by at least one ofink jet printing, offset printing, screen printing and laser marking ona dummy region of an upper substrate; forming a first alignment filmover an active region of the upper substrate; forming sealant along aperiphery of the active region of the upper substrate; and bonding theupper substrate to a lower substrate.

[0018] It is to be understood that both the foregoing description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this application, illustrate embodiment(s) of theinvention and together with the description serve to explain theprinciple of the invention.

[0020]FIG. 1 illustrates a disassembled perspective view of a part of arelated art TN liquid crystal display device.

[0021]FIG. 2 illustrates a sectional view of a related art LCD of a COTstructure.

[0022]FIG. 3 illustrates a plan view of a related art LCD of a COTstructure in FIG. 2.

[0023]FIGS. 4A to 4D are sectional views illustrating the steps of aprocess for forming alignment keys on an upper substrate in an LCDdevice according to an exemplary embodiment of the present invention.

[0024]FIG. 5 illustrates a plan view for forming alignment keys inaccordance with a first preferred embodiment of the present invention.

[0025]FIG. 6 illustrates a plan view for forming align keys inaccordance with a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings.

[0027] The present invention forms alignment keys, not by the complicatephoto-etching processes, but by ink jet printing, offset printing,screen printing, or laser marking, which requires no precision. Thepresent invention in which the alignment keys are formed by one of abovemethods is applicable to the COT or TOC type LCD, in which a lightshielding film is formed along an outer side periphery of a channelregion of a thin film transistor and an active region. Unlike thecomplicated processes described above, the method for fabricating an LCDof the present invention does not need high position accuracy nor highprecision in patterning the alignment keys. The alignment keys can bealignment film printing keys, bonding keys, and seal printing keys, andthe like.

[0028]FIGS. 4A to 4D are sectional views illustrating the steps of aprocess for forming alignment keys on an upper substrate in an LCDdevice according to an exemplary embodiment of the present invention.

[0029] Referring to FIG. 4A, an upper substrate 40 having an activeregion and a dummy region defined thereon is provided for forming afirst alignment film. The upper substrate 40 has neither a color filterlayer, nor a black matrix layer, formed thereon. This is because thecolor filter layer is formed on each of the pixel regions of a lowersubstrate, and a light shielding film is formed on an outer side of theactive region of the lower substrate for shielding a back light leakingfrom the outer side of the active region. Then, alignment keys areformed on the outer side of the active region of the upper substrate byusing ink jet printing, offset printing, screen printing, or lasermarking. The alignment keys formed in the dummy region 51 or 61 (seeFIG. 5 or 6) of the upper substrate will be cut away in a followingscribing process.

[0030] As shown in FIG. 4B, a first alignment film 41 is formed on theupper substrate 40. If neither the color filter layer nor the backmatrix layer on the upper substrate 40 are formed on the upper substrate40, no precise aligning is required in the formation of the firstalignment film 41 on the upper substrate 40 because there is no patternformed on the upper substrate 40. Thereafter, a rubbing process iscarried out in which no alignment keys are required.

[0031] As shown in FIG. 4C, after the first alignment film 41 is formed,sealant 42 is applied on the outer side of the active region of theupper substrate 40 spaced a distance away from the first alignment film41. In this instance, since the distance required between the firstalignment film 41 and the sealant 42 is approximately 0.5˜1 mm, highprecision is not required.

[0032] As shown in FIG. 4D, a process for bonding the upper substrate 40and the lower substrate 43 is carried out. In this instance, the lowersubstrate 43 has RGB color filter layers 44 on each of the pixel regionsin the active region, a light shielding film 45 on the outer peripheryof the active region, and a second alignment film 46 formed on theactive region inclusive including the color filter layer 44 and a regionextending therefrom. In the foregoing bonding of the upper substrate 40and lower substrate, high precision is not required because the distancebetween the sealant 42 on the upper substrate 40 and the secondalignment film 46 on the lower substrate 43 is approximately 0.5˜1 mm.Moreover, the precision required in the scribing for a plurality of cellunits formed on a large substrate is at a level of +0.5 mm, which is nothigh.

[0033] A relatively low precision is required in the processes offorming the first alignment film 41, positioning the sealant 42 on theupper substrate 40, and bonding of the upper and lower substrates 40 and43. Thus, there is no need for high precision in these processes thatrequires high precision alignment keys, such as an alignment filmprinting key, a seal printing key, and a bonding key in the dummy regionon an outer side of the active region of the upper substrate 40. Inother words, a sub-μm alignment precision like used in the formation ofthe thin film transistor of the related art is not necessary for theseprocesses. A level of 1 mm unit (1000 μm) precision is sufficient forthese processes in accordance with exemplary embodiments of theinvention. Accordingly, the complicate patterning technique of thephoto-etching and the like in the related art are not necessary.

[0034] In exemplary embodiments of the present invention, the alignmentkeys are fabricated by simple processes, such as ink jet printing,offset printing, screen printing, or laser marking, using low priceapparatuses. A process for forming the alignment keys using two of theabove methods will be described in reference to FIG. 5 and FIG. 6. FIG.5 illustrates a plan view for forming alignment keys in accordance witha first preferred embodiment of the present invention and FIG. 6illustrates a plan view for forming alignment keys in accordance with asecond preferred embodiment of the present invention. The alignment keys53 in FIG. 5 and alignment keys 63 of FIG. 6 are respectively formed ona dummy region 51 of the upper substrate 50 in FIG. 5 and a dummy region61 of the upper substrate 60 in FIG. 6.

[0035] Referring to FIG. 5, in the formation of the alignment keys byink jet printing, ink is dropped on desired parts of the dummy region 51of the upper substrate 50 upon reception of a signal to dispense inkusing an ink jet apparatus having an ink jet head 56. The ink jetapparatus includes a vertical head driver 54 c with a shaft 55 formoving the ink jet head 56 in up/down direction with respect to theupper substrate 50, and upper head driver 54 a and lower head driver 54b for moving the ink jet head 56 and the vertical head driver 54 c inleft and right directions, to form the alignment keys 53. Instead of theabove ink jet apparatus, the alignment keys can be formed by usingconventional ink jet apparatuses.

[0036] In addition or in the alternative, alignment keys can be formedby offset printing. Desired patterns are engraved in a copper plate 66,as shown in FIG. 6, for printing on the dummy region 61 of the uppersubstrate 60. The copper plate 66 is used to print the alignment mark byusing an offset printing apparatus having an ink dispensing roll 65 witha shaft 67 for moving the copper plate 66 in left and right directions,and an upper head driver 64 a and a lower head driver 64 b for movingthe copper plate 66 and the ink dispensing roll 65 in left and rightdirections together with the shaft 67. Instead of the above offsetprinting apparatus, the alignment keys can be formed by usingconventional offset printing apparatuses.

[0037] In conventional offset printing, ink is transferred from a platesurface to a rubber blanket, and therefrom to a place intended to printthereon. For example, a conventional offset printing apparatus has anassembly of three cylinders including a plate cylinder for winding aprinting plate, a rubber blanket, and an impression cylinder, and has awetting device for preventing attachment of the ink on non-scanningpart. Most of the conventional offset printing apparatuses are rotarypresses. Further, there are multi-color press which can print more thantwo colors. Furthermore, there are presses that can print on two sidesof an item.

[0038] In another alternative or in addition, alignment keys can beformed on a dummy region of an upper substrate by laser marking.Although not shown in the figures, a laser beam is directed onto thedummy region of the upper substrate so as to engrave the alignment keysof intended size and pattern. In yet another alternative or in addition,alignment keys can be formed on a dummy region of an upper substrate byscreen printing. Although not shown in the figures, the align keys canbe formed on the dummy region of the upper substrate using conventionalscreen printing processes.

[0039] Conventional screen printing processes includes printing with ascreening material, such as silk, nylon, tetoron fabric, stainless steelplate, or glass plate that is placed on and fastened to a frame. Squamaeis then formed either manually by eye or by a photo-chemical method onthe screening material. Printing is done by pouring printing ink intothe frame and applying pressure to an inside surface of the screen witha tool called a squeegee. The squeegee is moved so that the ink passesthrough parts of the screening material without the squamae to print theink on a material to be printed under the screening material. Screenprinting enables printing not only on a flat object, but also on a sideof cylinder, or conical body, direct adhesive printing. Further, screenprinting can be done on a flexible material, such as paper or cloth.Furthermore, screen printing can be done on a hard plate, such as metal,or hard plastic as well as on fragile material, such as glass due to itslow printing pressure. Moreover, the screen printing can print, to havea thick printed ink layer, on a unique shaped body (cup, bottle,industrial components, and the like), extra large sized poster, placard,various displaying materials, printed circuit board of electronicproducts, and even printing of micron precision.

[0040] As has been described, the method for fabricating a liquidcrystal display device according to an exemplary embodiments of thepresent invention has at least the following advantages. First, therequirements for position accuracy and pattern precision are not highfor forming the alignment keys on the upper substrate. Second, thealignment keys can be formed easily by using simple fabrication methods.That is, the fabrication process can be simplified because the alignmentkeys can be formed by a method that is less complicated and with theprecision of about 1 mm.

[0041] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the a method for fabricatinga liquid crystal display device of the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for fabricating a liquid crystal displaydevice comprising the steps of: forming alignment keys by at least oneof ink jet printing, offset printing, screen printing and laser markingon a dummy region of an upper substrate; forming a first alignment filmover an active region of the upper substrate; forming sealant along aperiphery of the active region of the upper substrate; and bonding theupper substrate to a lower substrate.
 2. The method as claimed in claim1, wherein the lower substrate includes: an active region having pixelregions defined therein; RGB color layers in the pixel regions; a lightshielding film on a periphery of the active region of the lowersubstrate; and a second alignment film on a part of the lower substrateopposite to the first alignment film on the upper substrate.
 3. Themethod as claimed in claim 1, wherein a distance between the sealant onthe upper substrate and the first alignment film is approximately 0.5˜1mm.
 4. The method as claimed in claim 2, wherein a distance between thesealant on the upper substrate and the second alignment film isapproximately 0.5˜1 mm.
 5. The method as claimed in claim 1, whereinforming alignment keys by ink jet printing includes dropping ink ondesired parts of the dummy region of the upper substrate upon receptionof a signal to dispense ink using an ink jet apparatus, wherein the inkjet apparatus has an ink jet head, a vertical head driver with a shaftfor moving the ink jet head in up/down directions with respect to theupper substrate, and upper and lower head drivers for moving the ink jethead, and the vertical head driver in left and right directions.
 6. Themethod as claimed in claim 1, wherein forming alignment keys by offsetprinting includes printing desired patterns engraved in a copper plateon the dummy region on the upper substrate while moving the copper platein left, and right directions by using an offset printing apparatus,wherein the offset printing apparatus has the copper plate with thedesired patterns engraved therein, an ink dispensing roll with a shaftfor moving the copper plate in left and right directions, and an upperhead driver and lower head driver for moving the copper plate and theink dispensing roll in left and right directions together with theshaft.
 7. The method as claimed in claim 1, wherein forming alignmentkeys by laser marking includes controlling the movement of a laser onthe dummy region of the upper substrate so as to inscribe the alignmentkeys.
 8. The method as claimed in claim 1, wherein the alignment keysare at least one of an alignment film printing key, a bonding key, and aseal printing key.
 9. The method as claimed in claim 1, wherein formingthe alignment keys has a precision at a level of 1 mm unit.